Heng Tsai

WEATHERING SEQUENCES OF CLAY MINERALS IN SOILS ALONG A SERPENTINITIC TOPOSEQUENCE

There has been limited research on clay mineral transformation in serpentinitic soils under humid tropical conditions. In this study, four soil pedons were selected along a toposequence from the summit (Entisol), shoulder (Vertisol), backslope (Alfisol) to footslope (Ultisol) positions to explore the contributions and the significance of landscape and weathering status of serpentinitic rock with regard to clay mineral transformations in eastern Taiwan. Experimental results indicated that the large amount of dithionite-citrate-bicarbonate-extractable Fe (Fed) and clay in the subsurface horizon were mainly caused by the strong leaching potential from intensive rainfall and weathering of the fine-grained parent rocks. The clay mineralogy reflected the clear weathering trend of the soils along the toposequence: (1) the soils on the summit and shoulder contained smectite and serpentine, which are predominant in the young soils derived from serpentinitic rocks; and (2) vermiculite gradually increased in the relatively old soils on backslope and footslope. The mineralogical transformations observed along the toposequence indicated that chlorite and serpentine, initially present in the Entisol on the summit, weather into smectite and interstratified chlorite-vermiculite in the intermediate soil on the shoulder under strong leaching and oxidizing conditions. Furthermore, vermiculite formed as the major weathering product of chlorite and smectite in the soil developed on the backslope. In addition to vermiculite, kaolinite and quartz formed in the soils on the footslope with the greatest concentration of Fed along the toposequence.

A river terrace soil chronosequence of the pakua tableland in central Taiwan

Soils science and fluvial geomorphology meet on the subject about flights of river terraces whose treads rise like stairs above active channels. The Pakua tableland in central Taiwan consists of widely unpaired terraces, including six levels in altitude formed by successive river incision. The terraces serve as geomorphic markers to gauge the differential or absolute deformation of the land surface but such application is hampered by the absence of well-documented ages. Six representative soil pedons were selected from the six levels of terraces in the tableland (from the highest pedon PK-1 to lowest pedon PK-6) to characterize the soil properties in a chronosequence and to relate the pedogenic processes in the major terraces to the formation and evolution of the landscape in the study area. The soil morphological, physical, and chemical properties as well as the clay mineral variation showed that pedogenic intensity is strongly dependent on the terrace levels with varying formation age. The soils can be divided into three domains as Hapludox for pedon PK-1, Paleudult (or Hapludult) for pedons PK-2, PK-3, PK-4, and PK-5, and Dystrudept for pedon PK-6, based on Soil Taxonomy. The degree of soil development increases with altitude in a sequence from PK-1 to PK-6 forming a postincisive type of soil chronosequence in accord with the evolution of the geomorphic surface by successive river incision in the study area. Based on the crystallinity ratios of free iron, the soils give an estimated age of 40-400 ka for the river terraces of the tableland.

Geochemical Element Differentiation in Serpentine Soils From the Ophiolite Complexes, Eastern Taiwan

The vertical distribution of 14 geochemical elements (Si, Al, Ti, K, Na, Ca, Mg, Fe, Mn, Cu, Cr, Ni, Pb, and Zn) in nine serpentine pedons for three ophiolite complexes was investigated. The pedons are located at Wu-Mau, Tong-An, and Shih-Tao mountains in the Coastal Range, eastern Taiwan. The objective of this study was to discriminate the parent material inheritance and pedogenic factors by comparison of the element concentrations based on principal component (PC) analysis. From the experimental results, the relative abundance of major elements was as follows: Si > Fe > Mg > Al, with lesser concentrations of Ti, K, Na, and Ca. In addition, Cr, Mn, and Ni contents in these soils were higher than in soils formed from other parent materials, with considerable variation between pedons on different landscapes, which reflected the ophiolite differences and degrees in chemical weathering of the associative serpentinitic rocks. The contents of pairs of Si-Fe, Si-Mn, Si-Pb, and Si-Zn showed significantly (P < 0.05) positive correlations. The close relations among contents of Fe, Mn, Cr, and Ni were observed herein. According to the eigenvalue in PC analysis, the elements most responsible for separation along the PC-1 axis were Si, Fe, Mn, Zn, Mg, and Ca, whereas Cr and Ni were the trace metals responsible for separation along the PC-2 axis. This result reflected the parent material differences in igneous rock composition and the degree of serpentinization between ophiolite complexes.

Geomorphological and paleoclimatic implications of soil development from siliceous materials on the coral-reef terraces of Liuchiu Island in southern Taiwan

Liuchiu Island is an uplifted coral-reef island located off southwestern Taiwan. A total of four soil pedons, labeled as LC-1 and LC-2 from the Holocene terraces and LC-3 and LC-4 from the Pleistocene terraces, were sampled on the island for this work. These soils were siliceous, and were characterized by enrichment of clay and free iron (Fed). According to Soil Taxonomy, pedons LC-3 and LC-4 were classified as Paleudults and pedons LC-1 and LC-2 were Dystrudepts. The soil properties showed progressive changes from pedon LC-1 to pedon LC-4 in morphology, physical and chemical properties, and clay mineralogy. The contents of total Fe and dithionite-citrate-bicarbonate extractable Fe were significantly higher in pedons LC-3 and LC-4 with high weathering degree than in pedons of LC-1 and LC-2 with less weathering degree. Enrichment of kaolinite and gibbsite in pedons LC-3 and LC-4 also suggested high chemical weathering degree of the soils. The estimated soil ages for all studied pedons were consistent with their degrees in pedogenesis, where pedons LC-3 and LC-4 were located at older terraces and pedons LC-1 and LC-2 were located at younger terraces. Namely, it complied with the geologic interpretation of the continuous and simultaneous uplift and tilt of the island over time. Instead of the in situ weathering from the underlying coral reef limestone, all soils developed from siliceous parent materials deposited onto the surfaces. The SiO2/Al2O3 ratios of soils indicated a component of loess may have been incorporated from continental China as part of the parent material, which confirmed a climate change of strong monsoons or severe dust storms occurred before the Holocene. However, soil development increased by the subsequent warm and humid climates of the interglacial stage over time.

The soils of Taiwan.

ed from Chen and Huang (1991) 4.2 Profile Characteristics 43

Asian Anthroscapes: China and Taiwan

Anthroscapes are important in the assessment of human-induced land degradation. In case of soils affected by land use changes in large-scale farming, the anthropogenic processes have been created in China for over a few thousands years. The Chinese started to use soil for crop production as early as in the Stone Age. Soil properties in China and Taiwan have been greatly influenced by various anthropogenic processes to form anthroscapes. Paddy soils or rice-growing soils are more than 30 million hectares, distributed in China and Taiwan, which are one of the world’s most important soil resources for food production. According to the Chinese Soil Taxonomy [ISS/CAS (Institute of Soil Science, Chinese Academy of Sciences) (ed.) (2001) Chinese Soil Taxonomy. Science Press, Beijing, 203p], Anthrosols meet the requirements of the combination of horizons, including anthrostagnic, fimic, siltigic and cumulic epipedons and the hydragic horizon. The changes of soil characteristics after planting rice are degradation of soil organic matter (SOM), redistribution of exchangeable bases, translocation and segregation of iron (Fe) and manganese (Mn) by the cycling of reduction and oxidation processes, as well as decomposition and synthesis of clay minerals. The genetic horizons of paddy soils in China and Taiwan include a cultivated gray horizon, a plowpan, a percogenic horizon, and a plinthitic horizon or illuvial horizon with various redoximorphic features. The landscape position is the major factor to control the duration of saturation and reduction in the paddy soils of Taiwan. An anthrogleyic epipedon in Taiwan was also common in the surface horizon in paddy soils. Irrigation has induced the enrichment of the soil organic carbon (SOC) stock in the paddy soils. The free Fe contents of paddy profiles tend to be increased as the length of cultivation history increases in China, but this trend is much less profound in non-paddy profiles.

Evaluating Soil Dating by Amounts and Activity of Pedogenic Iron on Quaternary Terraces of Taiwan

As part of the island arc groups created by tectonic collision between the Eurasian and Philippine Sea Plates, Taiwan has many fluvial and marine terraces. However, the absence of absolute dating and geochronological information means that extracting information from deformed surfaces is problematic. Previous studies correlated terraces with date based primarily on geomorphic and pedogenic parameters, but the results were usually inconsistent and unsatisfactory. Other indices of soil age include the ratio of iron from crystalline pedogenic iron oxides to total iron [(Fed-Feo)/ Fet] and the activity of the pedogenic iron oxides (Feo/Fed). This study sampled ten representative soil pedons from four Quaternary fluvial terraces of different geological environments in Taiwan with similar parent materials. The objectives of this study are to characterize soil properties of different fluvial terraces and use the crystallinity ratio of free iron oxides to estimate the age of these soils. This study classifies soils as Oxisols, Ultisols, and Inceptisols with udic soil moisture regime based on Soil Taxonomy and as Ferralsols, Acrisols and Cambisols based on the WRB (World Reference Base for Soil Resources) system. These soils are generally red, fine textured with clear clay coatings on ped surface, strongly acidic, and contain low organic carbon levels. These characteristics indicate strong chemical weathering and significant eluvial and illuvial processes of clay and iron in these soils. High kaolinite and gibbsite contents and the absence of smectite indicate high levels of leaching and desilification processes in the soil samples, reducing the amount of weatherable primary minerals. The iron activity ratio (Feo/Fed) and crystallinity ratio of free iron oxides [(Fed-Feo)/Fet] of subsurface (i.e. Bo, Bt, Bw) horizons may serve as a good indicator of soil development in Taiwanese terraces. Based on the relationship of iron activity ratio and crystallinity ratio of free iron oxides established with known soil ages, this study estimates approximate ages of 19-202 ka, 16 ka, 83-97 ka, and 13 ka for soils developed in the Pakua, Touliu, Chushan, and Laopi terraces. These results agree with extant knowledge of pedogenic processes and terrace formation in Taiwan. The correlation between the iron activity ratio and the crystallinity ratio of free iron oxides can be improved if more databases become available for soils from different landscapes, parent materials, and ages.